Continuous galvanizing line operations result in the production of considerable quantities of dross. Of the dross produced, some 80% to 90% is top dross or skimmings which float on the top surface of the bath of molten zinc alloy in the continuous galvanizing line coating pot. In such continuous galvanizing lines, several percent of the total zinc consumption may be lost to top dross. Although top dross can run from about 94% to about 99% total zinc, it is traditionally sold to zinc or zinc chemical producers at a considerably lower price than that paid for prime grade zinc. As a result, this represents a substantial economic loss to the galvanizer.
Chemical and metallographic examinations reveal the presence of three distinct phases in a typical sample of top dross. The top dross comprises a fibrous or sheet-like zinc oxide phase encapsulating large amounts of galvanizing zinc alloy in which an aluminum-iron-zinc alloy occurs as dispersed inclusions.
Prior art workers have made numerous attempts to recover zinc values from dross. Most of these attempts have been characterized by one or more drawbacks. Frequently prior art methods have been batch methods and have involved complex apparatus and difficult material handling problems including shaking, stirring, rotation or oscillation of the dross. Many prior art methods require a flux and/or tend to produce zinc fume, both of which present a pollution problem and/or a worker health problem.
U.S. Pat. No. 831,123 teaches a recovery apparatus comprising a fuel-fired brick furnace and a specially configured dross pan. The furnace is intended to bring the zinc values in the dross to the melting point whereupon they run out of the dross pan into a vessel by which they may be returned to the coating pot. U.S. Pat. No. 1,821,105 teaches apparatus for the collection of bottom dross by means of an endless chain of buckets. Zinc values are obtained through the use of a steam or air actuated cylinder to compress the dross through a perforated container. U.S. Pat. No. 2,433,615 utilizes a retort and a vaporization process to recover zinc from dross.
U.S. Pat. No. 2,463,468 again teaches a vaporization process wherein dross is melted under non-oxidizing conditions so as to evaporate zinc values therefrom. The zinc vapors are carried to a condenser and liquid zinc is recovered and cast into slabs. U.S. Pat. No. 2,481,591 relates to a method of treatment of aluminous dross. While an aluminum system is different from a zinc system, the reference is of interest as illustrating an exemplary dross recovery system utilizing a flux and rotation or oscillation of the treated dross. U.S. Pat. No. 2,701,194 sets forth a process for recovering zinc metal and its alloys from zinc dross utilizing a low melting point flux composition.
The process of removal of zinc values from slag taught in U.S. Pat. No. 2,795,500 includes the step of injecting a molten dross pool with a combustible mixture of liquid fuel and oxygen-containing gas. This reduces the reducible zinc compounds and the zinc is thereafter volatilized. Another vaporization system is set forth in U.S. Pat. No. 2,844,462. U.S. Pat. No. 3,198,505 is an example of a reference relating to a metal recovery apparatus for stirring dross in a drum. U.S. Pat. No. 4,003,559 sets forth apparatus for squeezing zinc from dross. Finally, U.S. Pat. No. 4,075,008 teaches the formation of a dross ingot placed on a permeable support and heated so as to melt zinc therefrom, the zinc being collected below the permeable support.
The method and means of the present invention contemplate the utilization of a continuous tapping, sloping hearth furnace which discharges the recovered zinc alloy directly into the coating pot. Both the apparatus and the process are simple, energy and cost efficient and involve reduced material handling. The process is free from zinc fuming, does not require the use of a flux and produces salable residue. No stirring, squeezing, shaking or the like is required.